Continuous casting nozzle for molten steel

ABSTRACT

A continuous casting nozzle for casting molten steel, wherein at least a surface layer of an inner bore of the continuous casting nozzle contacting with a molten steel is formed of a refractory comprising: an aggregate consisting essentially of alumina (Al 2 O 3 ) or an aggregate comprising alumina (A1 2 O 3 ) as its main ingredient and melting point thereof being at least 1800 degree centigrade: from 15 to 60 wt. % and roseki as a balance. The refractory is added thermosetting resin as binder, kneaded, formed, and sintered in an anti-oxidizing atmosphere.

FIELD OF THE INVENTION

The present invention relates to a continuous casting nozzle, in particular, a continuous casting nozzle which permits effective prevention of narrowing and clogging of the inner bore thereof through which molten steel passes in performing continuous casting of the molten steel containing aluminum such as aluminum-killed steel.

THE RELATED ART

A continuous casting nozzle for casting molten steel is used for the following purposes.

A continuous casting nozzle has a function of pouring molten steel from a tundish to a mold. In continuously casting molten steel, a continuous casting nozzle is used for such purposes as preventing the molten steel from being oxidized by contacting with the open air, preventing the molten steel from splashing when the molten steel is poured from a tundish to a mold, and rectifying the flow of the poured molten steel so as not entrap non-metallic inclusion and slag present near or on the mold surface into the cast steel strand.

A refractory material of a conventional continuous casting nozzle of molten steel comprises graphite, alumina, silica, silicon carbide or the like, for example. However, there are following problems when aluminum-killed steel or the like is cast with the use of the conventional casting nozzle.

In casting the aluminum-killed steel or the like, aluminum which is added as a de-oxidizer, reacts with oxygen existing in the molten steel to produce non-metallic inclusion such as alpha (α)-alumina or the like. In addition, when the molten steel flows through the nozzle, the aluminum in the molten steel reacts with oxygen in the open air to further produce alumina.

Therefore, in casting the aluminum-killed steel or the like, the non-metallic inclusion such as α-alumina adheres and accumulates onto the surface of the inner bore of the continuous casting nozzle, so that the inner bore is narrowed or clogged up in the worst case so as to make the stable casting thereof difficult. Furthermore, the non-metallic inclusion such as α-alumina adhered or accumulated onto the surface of the inner bore peels off or falls down, and the non-metallic inclusion thus peeled off or fell down is entrapped into the cast steel strand, thus degrading the quality of the cast steel strand.

In order to prevent the above-mentioned reduction or clogging of the inner bore of the nozzle caused by the non-metallic inclusion such as α-alumina, there has widely been used the method in which inert gas is ejected from the inner surface of the nozzle bore toward the molten steel flowing through the inner bore so as to prevent the non-metallic inclusion such as α-alumina existing in the molten steel from adhering or accumulating on the surface of the inner bore of the nozzle (for example, the method disclosed in Japanese Patent Publication No. Hei 6-59533/1994).

However, there are problems in the above-mentioned method in which the inert gas is ejected from the inner surface of the inner bore of the nozzle, as follows:

When a large amount of inert gas is ejected, bubbles produced by the inert gas is entrapped into the cast steel strand to cause pinholes in the cast steel strand, thus deteriorating the quality of the cast steel. On the other hand, when a small amount of inert gas is ejected, the sufficient effect of the inert gas is not obtained, and non-metallic inclusion such as the α-alumina is adhered and accumulated onto the surface of the inner bore of the nozzle, thus causing narrowing or clogging, in the worst case, of the inner bore.

In addition, it is substantially difficult to manufacture the nozzle which enable to uniformly eject the inert gas from the surface of the inner bore of the nozzle toward the molten steel flowing through the inner bore. Furthermore, when the casting is performed for a long period of time, it becomes gradually difficult to stably control the amount of ejected inert gas, since the refractory material of the continuous casting nozzle degrades. As a result, the non-metallic inclusion such as α-alumina adheres and accumulates onto the surface of the inner bore of the nozzle in such manner that the inner bore is narrowed or eventually clogged up.

The clogging of the nozzle by the non-metallic inclusion, particularly alumina (Al₂O₃) inclusion is deemed to be caused as follows:

(1) Aluminum in the molten steel is oxidized by the entrapped air which passes through a joint portion of the nozzle refractory and the refractory structure per se to produce alumina, or silica in the refractory including carbon is reduced to produce SiO which supples oxygen to produce alumina.

(2) Alumina inclusion is produced by diffusion and cohesion of the alumina produced in the above process.

(3) Graphite and carbon on the surface of the inner bore of the nozzle are taken away in such manner that the feature of the surface of the inner bore becomes rough, and thus the alumina inclusion is apt to accumulate on the rough surface of the inner bore.

There is proposed a nozzle as a remedy to solve the above problem, in which a non-oxide raw material (SiC, Si₃N₃, BN, ZrB₂, Sialon, etc.) that has a low reactivity with aluminum oxide is added to alumina-graphite refractory, or a nozzle consisting of the above non-oxide material itself (for example, refer to Japanese Patent Publication No. Sho 61-38152/1986).

However, it is not practical to add the above non-oxide material to the widely used alumina-graphite refractory, because the effect of preventing adhesion is not recognized unless a large amount of the non-oxide material is added, and furthermore, the corrosion resistance thereof is deteriorated when a large amount of the non-oxide material is added thereto.

Also, the nozzle consisting essentially of the non-oxide material is not suitable for practical use, since the material cost and manufacturing cost are expensive, while the substantial effect of preventing adhesion may be expected.

There is further proposed a nozzle, the refractory thereof comprising graphite-oxide raw material containing CaO, in which an oxide raw material containing CaO (CaO.ZrO₂, CaO.SiO₂, 2CaO.SiO₂, and the like) produces by a reaction of CaO with Al₂O₃ a low-melting-point material which is easily separated from the molten steel (for example, refer to Japanese Patent Publication No. Sho 62-56101/1987).

However, since the reactivity of CaO with Al₂O₃ is apt to be influenced by a temperature condition of the molten steel in casting, it is difficult to effectively produce the low-melting-point material. In addition, a large amount of CaO is required to supply when a large amount of Al₂O₃ inclusion is contained in the steel. However, it is difficult to contain sufficient amount of CaO in the refractory of the nozzle, since spalling resistance and corrosion resistance thereof are deteriorated. Furthermore, zirconia (ZrO2) is difficult to be separated from the molten steel, since zirconia in the aggregate flowing into the molten steel from the refractory has a high specific gravity so that zirconia stays in the molten steel.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a continuous casting nozzle which may prevents alumina inclusion from adhering and accumulating on the inner surface of the nozzle, and prevents the inner bore of the nozzle from being narrowed and clogged so as to enable a stable casting, by means of forming a glass layer on the surface of the inner bore of the nozzle when the nozzle is used, thereby preventing air from being entrapped through refractory structure thus not to produce alumina, and in addition, smoothing the surface of the inner bore of the nozzle.

The first embodiment of the present invention is a continuous casting nozzle for casting molten steel, wherein at least a surface layer of an inner bore of said continuous casting nozzle contacting with a molten steel is formed of a refractory comprising:

an aggregate consisting essentially of alumina (Al₂O₃) or an aggregate comprising alumina (Al₂O₃) as its main ingredient and melting point thereof being at least 1800 degree centigrade (°C.): from 15 to 60 wt. %; and

roseki as a balance.

The second embodiment of the present invention is a continuous casting nozzle for casting molten steel, wherein at least a surface layer of an inner bore of said continuous casting nozzle contacting with a molten steel is formed by a process in which binder is added to a refractory material comprising 15 to 60 wt. % of an aggregate consisting essentially of alumina (Al₂O₃), or an aggregate comprising alumina (Al₂O₃) as its main component and melting point thereof being at least 1800° C. and roseki as a balance, and then said refractory material with said binder added is kneaded, formed, and sintered in an anti-oxidizing atmosphere.

The third embodiment of the present invention is a continuous casting nozzle for casting molten steel, wherein a mixing weight ratio of roseki having average grain diameter of up to 250 μm is up to 60 wt. % relative to a total content of said roseki.

The fourth embodiment of the present invention is a continuous casting nozzle for casting molten steel, wherein said roseki comprises pyrophyllite (Al₂O₃.4SiO₂.H₂O) as its main component.

The fifth embodiment of the present invention is a continuous casting nozzle for casting molten steel, wherein said roseki comprising roseki which is calcinated at a temperature of at least 800° C. so as to remove crystal water therein.

The sixth embodiment of the present invention is a continuous casting nozzle for casting molten steel, wherein said binder comprises a thermosetting resin.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically shows a longitudinal cross section of a nozzle according to the present invention in which the refractory of the invention is formed at the surface layer of the inner bore of the nozzle contacting with molten steel.

FIG. 2 schematically shows a longitudinal cross section of a nozzle according to the present invention in which the refractory of the invention is formed both at the surface layer of the inner bore of the nozzle and the lower part (a part immersed in the molten steel) of the nozzle.

FIG. 3 shows Table 1.

PREFERRED EMBODIMENTS OF THE INVENTION

The most important feature of the casting nozzle of the present invention resides in that roseki is used as its main ingredient of the refractory material of the nozzle and graphite which is often used in the conventional nozzle is not contained.

Graphite contained in the conventional nozzle reacts with silica contained in the nozzle, when the nozzle is used in operation, as follows:

SiO₂(S)+C(S)=SiO(g)+CO(g)

3SiO(g)+2Al=Al₂O₃(S)+3Si

3CO(g)+2Al=Al₂O₃(S)+3C

According to the above reactions, the silica in the nozzle is decomposed to produce SiO(g) and CO(g), which become an origin to supply oxygen to the molten steel, and thus supplied oxygen reacts with aluminum in the molten steel to produce Al₂O₃.

However, the particles of roseki does not decompose even if carbon coexists in the molten steel. In particular, the SiO₂ contained in pyrophyllite (Al₂O₃.4SiO₂.H₂O) or the like which is the main mineral of the roseki is stable. The above-mentioned fact is acknowledged in the following manner: a briquette comprising the roseki, resin powders and carbon powders was formed and buried in a breeze, and heat-treated at a temperature of 1500° C. for 24 hours, and then the thus treated briquett was investigated with a microscope to find that the particles did not decay and bubbles were not produced.

The conventional refractory material with 10 wt. % of graphite added has a thermal conductivity of 9.8 (kcal/m/hr/°C.), whereas the refractory material of the invention which does not contain graphite has such a low thermal conductivity as 2.4 (kcal/m/hr/°C.), and excellent heat resistance. As a result, the refractory material of the invention shows effective prevention of metal from being adhered or non-metallic inclusion such as α-alumina (Al₂O₃) from being precipitated.

In addition, in the conventional nozzle containing graphite, when the graphite is oxidized, the smoothness of the surface of the inner bore is lowered. Furthermore, the molten steel flowing through the inner bore of the nozzle produces turbulence so as to cause the non-metallic inclusion such as α-alumina to accumulate on the inner surface of the nozzle. However, since the surface layer of the inner bore of the nozzle of the invention does not contain graphite, the smoothness of the surface of the inner bore is not lowered. In other words, concave and convex portions are not formed on the inner surface of the nozzle of the invention, thus the non-metallic inclusion such as α-alumina is not accumulated on the inner surface of the nozzle of the invention.

The half-melting temperature of the roseki is around 1500° C., so that it melts at the surface of the inner bore of the nozzle contacting with the molten steel to form a glass coat in such manner that the structure of the surface of the inner bore becomes to be smooth and air is prevented from being entrapped through a refractory structure.

The above-mentioned fact was acknowledged in the following manner: the refractory comprising alumina-roseki with graphite added is heat-treated at a temperature of 1500° C. for 1 hour in the oxidizing atmosphere, and the permeability thereof was investigated to find out to be about 6.5×10⁻⁴ darcy, whereas the refractory comprising alumina-roseki without graphite was heat-treated at a temperature of 1500° C. for 1 hour in the oxidizing atmosphere, and the permeability thereof was investigated to find out to be about 1.0×10⁻⁴ darcy, thus the permeability is lowered.

The roseki is contained in the refractory of the surface layer of the inner bore of the nozzle of the invention as a balance, i.e., the remaining ingredient of the refractory. To actively form the glass coat on the surface of the inner bore when used as a continuous casting nozzle, a mixing weight ratio of the roseki in the surface layer of the inner bore of the nozzle is preferably at least 40 wt %. Also, it is preferable that the mixing weight ratio of the roseki in the surface layer is up to 85 wt %, because with the mixing weight ratio of the roseki over 85 wt %, degree of softening deformation is large, and corrosion resistance against molten steel is deteriorated.

In the continuous casting nozzle of the present invention, the refractory of the surface layer of the bore of the nozzle comprises roseki and 15 to 60 wt. % of an aggregate consisting essentially of Al₂O₃ or an aggregate comprising Al₂O₃ as its main ingredient and melting point thereof being at least 1800 degree centigrade. As the aggregate comprising Al₂O₃ as its main ingredient, spinel (MgO.Al₂O₃) is used which has a function to provide the surface layer of the inner bore of the nozzle with strength and corrosion resistance.

Three kinds of roseki may be used as the above-mentioned roseki, that is pyrophyllite roseki, kaolin roseki, and sericite roseki. The pyrophyllite roseki with refractoriness from SK29 to SK32 (SK is a Japanese Standard for refractoriness) is the most suitable, because the roseki is half-molten when the surface layer of the inner bore contacts with the molten steel to form a glass layer and the erosion resistance thereof against the molten steel is excellent. On the other hand, the kaolin roseki has a greater refractoriness from SK33 to SK36, and the sericite roseki has a smaller refractoriness from SK26 to SK29, both of which are not preferable.

It is preferable to use the roseki calcinated at a temperature at least 800° C. to vanish (remove) crystal water. The reason for using the above roseki is that when the nozzle containing not calcinated roseki is formed and sintered, the crystal water is released from the roseki at a temperature within a range of from 500 to 800° C. in sintering thereof, and then, the formed body may cracks by virtue of an unusually large thermal expansion coefficient.

It is preferable that a mixing weight ratio of roseki having average grain diameter of up to 250 μm should be up to 60 wt. % relative to the total content of the roseki, because when a mixing weight ratio of roseki having average grain diameter of up to 250 μm is over 60 wt. %, structural defects such as lamination are inclined to be produced in molding and softening deformation of roseki particles is inclined to happen when used in operation as a continuous casting nozzle.

Roseki comprising pyrophyllite (Al₂O₃.4SiO₂.H₂O) as its main component may be more preferably contained in the refractory within a range from 65 to 85 wt. %. In the refractory comprising an aggregate consisting essentially of alumina (Al₂O₃), or an aggregate comprising alumina (Al₂O₃) as its main ingredient and the melting point thereof being at least 1800 degree centigrade, for example, spinel within a range of from 15 to 60 wt. %, and roseki as the balance, roseki particles are not discomposed, so that oxygen is not supplied into the molten steel, contrary to SiO2.

The half-melting temperature of the roseki is about 1500° C. which is almost the same temperature as the casting temperature of molten steel. Accordingly, the roseki melts at the surface of the inner bore of the nozzle contacting with the molten steel to form a glass coat in such manner that the structure of the surface of the inner bore is smoothed and air is prevented from being entrapped through a refractory structure, thus preventing alumina (Al₂O₃) and metal from adhering thereon.

Thermosetting resin, for example, phenol resin, furan resin or the like is added as a binder within a range of from 5 to 15 wt. % to the above-mentioned material comprising the roseki and the aggregate, and then, a formed body of a nozzle is prepared and sintered. It is preferable that the above-mentioned formed body is prepared by the CIP (Cold Isostatic Pressing) process, considering that the formed body is uniformly compressed through the CIP process. The sintering temperature is preferably within a range of from 1000 to 1300° C. Reduction atmosphere, namely the anti-oxidizing atmosphere is preferable than the oxidizing atmosphere as the sintering atmosphere because the added thermosetting resin is not oxidized in the reduction atmosphere.

The continuous casting nozzle for molten steel according to the present invention is described in detail with reference to the drawings.

FIG. 1 schematically shows a vertical sectional view of the immersion nozzle for continuous casting according to the present invention. The continuous casting nozzle 10 is placed between a tundish and a mold, and used in operation as an immersion nozzle for pouring the molten steel from the tundish to the mold.

As shown in FIG. 1, a surface layer 2 of the inner bore 1, through which the molten steel flows, of the immersion nozzle 10 is formed by a refractory having the chemical composition as described above. The remaining part of the nozzle 3 is formed by a conventional alumina-gaphite refractory.

The dimensions of the nozzle are about 1 m in total length, about 60 mm in diameter of the inner bore, 160 mm in outer diameter of the nozzle, and about 50 mm in thickness. The thickness of the surface layer of the inner bore made of the refractory in the present invention is from about 2 mm to about 15 mm. The above-mentioned dimensions are shown as the example, and the nozzle of the present invention is not limited to the above dimensions. More specifically, the dimensions vary in accordance with the composition of the molten steel to be cast, and the size of the cast strand.

FIG. 2 schematically shows a longitudinal cross section of a nozzle in which the surface layer of the bore of the nozzle and the lower part (a part immersed in the molten steel) of the nozzle is made of the refractory according to the present invention. In either case, alumina which clogs the inner bore of the nozzle is collected at the inner bore in the lower part of the nozzle. According to the immersion nozzle of the present invention, the non-metallic inclusion such as alumina or the like is prevented from adhering and accumulating on the surface layer portion 2 of the inner bore 1. The present invention is described by the examples.

EXAMPLES

Phenol resin in the state of powder and liquid was added in an amount within a range of from 5 to 10 wt. % to each of nine pieces of mixed materials having a different chemical composition, and kneaded. From the thus kneaded materials, a first formed body (hereinafter referred to as the “formed body 1”) with dimensions of 30 mm×30 mm×230 mm for investigating an amount of adhesion of non-metallic inclusion such as alumina and corrosion resistance against the molten steel, a second formed body (hereinafter referred to as the “formed body 2”) with dimensions of 50 mmφ×20 mm for investigating permeability, and a third formed body (hereinafter referred to as the “formed body 3”) with dimensions of 100 mm in outer diameter, 60 mm in inner diameter and 250 mm in length for investigating spalling resistance, were respectively prepared, and then the respective bodies were sintered in reducing atmosphere at a temperature within a range from 1000 to 1200° C. to prepare samples No. 1 to 9.

The samples No. 1 to 5 (hereinafter referred to as the “sample of the present invention”) have the chemical compositions within the scope of the present invention, and the samples No. 6 to 9 (hereinafter referred to as “sample for comparison”) have the chemical compositions out of the scope of the present invention.

Physical properties (porosity and bulk density) for each of the above-mentioned samples of the present invention Nos. 1 to 5 and the samples for comparison Nos. 6 to 9 are shown in Table 1. The spalling resistance of the samples of the present invention Nos. 1 to 5 and the samples for comparison Nos. 6 to 9 prepared by the formed body 3 were investigated after being heated at a temperature of 1500° C. for 30 minutes in an electric furnace and then rapidly cooled by water. The results thereof are shown in Table 1.

An erosion ratio (%) and an amount of adhesion of non-metallic inclusion such as alumina of the samples of the present invention Nos. 1 to 5 and the samples for comparison Nos. 6 to 9 prepared by the formed body 1 were investigated after being immersed in molten steel having a temperature of 1520° C. for 180 minutes, which contains aluminum within a range from 0.02 to 0.05 wt %. The results thereof are also shown in Table 1.

The permeability of the samples of the present invention Nos. 1 to 5 and the samples for comparison Nos. 6 to 9 prepared by the formed body 2 were investigated after being heated at a temperature of 1500° C. for 60 minutes in an electric furnace and then cooled. The results thereof are also shown in Table 1.

As is clear from Table 1, all of the samples of the present invention show excellent spalling resistance, and the non-metallic inclusion such as alumina are not adhered in spite of the low erosion rate, thereby effectively preventing narrowing or clogging of the continuous casting nozzle for molten steel.

Also, it is possible for the samples of the present invention to prevent air from being entrapped through the refractory in practical use because of small permeability.

On the other hand, it is obvious that the sample for comparison No. 6 is remarkably inferior in the corrosion resistance against the molten steel, although a small amount of alumina adheres due to a large amount of roseki content.

As for the sample for comparison No. 7, the amount of adhesion of alumina is remarkably large, because it contains simple alumina (Al₂O₃) and simple silica (SiO₂), in which SiO₂ decomposes to supply oxygen into the steel. The sample for comparison No. 8 is remarkably inferior in spalling resistance, has a high permeability and shows adhesion of large amount of non-metallic inclusion such as alumina, due to the small amount of roseki content and the large amount of alumina (Al₂O₃), inspite of the mineral supplying oxygen into the molten steel is removed.

As for the sample for comparison No. 9 which comprises graphite, roseki and alumina (Al₂O₃), the amount of alumina adhesion is slightly large and the amount of metal adhesion is large when the temperature of the molten steel is as low as 1520±10° C. due to the containing of graphite.

Therefore, with the use of the continuous casting nozzle according to the present invention, it is possible to perform stable casting without deterioration of the refractory structure, while preventing narrowing or clogging of the bore caused by the non-metallic inclusion such as alumina.

According to the present invention, when 300 ton per charge of a low carbon aluminum killed steel was continuously cast by 2 strand slab caster, 5 to 7 charges of the steel were cast with the use of single nozzle without clogging.

Meanwhile, according to the conventional nozzle, when 2 to 4 charges of the steel were cast, the nozzle was clogged up and the casting had to be interrupted. As mentioned above, the effect of this invention was remarkable. 

What is claimed is:
 1. A continuous casting nozzle for casting molten steel, wherein at least a surface of an inner bore of said continuous casting nozzle contacting with a molten steel is formed of a refractory comprising: an aggregate consisting essentially of alumina (Al₂O₃) or an aggregate comprising alumina (Al₂O₃) as its main ingredient and melting point thereof being at least 1800 degree centigrade (°C.): from 15 to 60 wt. %; and roseki as a balance, wherein a graphite is not contained in said aggregate.
 2. A continuous casting nozzle according to claim 1, wherein a mixing weight ratio of roseki having average grain diameter of up to 250 μm is up to 60 wt. % relative to a total content of said roseki.
 3. A continuous casting nozzle according to claim 1, wherein said roseki comprises pyrophyllite (Al₂O₃.4SiO₂.H₂O) as its main component.
 4. A continuous casting nozzle according to claim 1, wherein said roseki comprising roseki which is calcinated at a temperature of at least 800° C. so as to remove crystal water therein.
 5. A continuous casting nozzle according to claim 1, further compnsing a binder having a thermosetting resin.
 6. A continuous casting nozzle according to claim 5, wherein said refractory material with said binder added is kneaded, formed, and sintered in an anti-oxidizing atmosphere. 